The present invention relates to methods for applying a desired shape to archwires to be used in orthodontic appliances for the straightening of teeth, and more particularly, to the automated design and manufacture of customized fixtures allowing an archwire made from shape memory alloy to be heat-treated while being inserted into said fixture.
The typical approach to straighten or align teeth of a patient is to bond brackets onto the teeth and, in succession, insert a series of wires with increasing stiffness into the bracket slots. In general, the shape of each wire reflects already the curve along which the bracket slots are to be located at the successful end of treatment. The advantage of such an approach is that the shapes of all wires of the series are substantially equal.
Depending on the severeness of the malocclusion, that is how far the teeth are away from their desired position, the shape of the wire will show significant deviations from the shape dictated by the current location of the bracket slots. The insertion of such a wire is therefore only possible if the wire is made from a very flexible material. Typically, wires made from so-called shape memory alloys (SMA) are therefore used in the initial phase of treatment.
Shape memory alloys used in the orthodontic profession show two different grades of stiffness below and above a so-called transformation temperature. Below the transformation temperature, which will typically be in a range between 25° and 40° Celsius, the wires are very soft, while when heated above the transformation temperature, the stiffness and thus the forces developed as a result from wire deflection increase significantly. Furthermore, the wire shows a “memory” of its shape. Even when seriously deformed at a temperature below the transformation temperature, including plastic deformations to a certain degree, the wire will develop significant forces in order to return to its original shape when it is heated above the transformation temperature.
SMA wires can easily be inserted into severe malocclusions. During insertion, the orthodontist does not have to be afraid of “overbending” the wire and permanently deforming it, since when heated above the transformation temperature by the body temperature of the patient (and additionally by warm food or drinks consumed by the patient), the wire will “remember” its original shape and drive the teeth to the desired location.
The significant disadvantage of SMA wires in orthodontic treatment is that they can hardly be modified by the orthodontist in order to adapt them to patient specific conditions. To permanently reshape a wire of SMA, the wire has to be exposed to a very high temperature of typically well above 300° Celsius, depending on the specific alloy. Since the shape memory effect is present while the wire is being heated in order to reach the desired temperature, it has to be mechanically constrained to the desired shape during the heat treatment. Only after the heat treatment is finished, the wire can be released and from then on “remembers” its new shape.
For the majority of orthodontic treatments the brackets are bonded to the outside of the teeth (labial side). Since labial tooth surfaces are more or less generic, standardized arch shapes can be used with good results. However, an increasing number of cases are treated using brackets that are bonded to the inside of the teeth (lingual side). Lingual brackets have great aesthetic advantages, since they are virtually invisible. The significant disadvantage is that the lingual tooth surfaces are by far not as generic as the labial surfaces. Because of this, for lingual brackets the need for customized Archwire shapes is even higher than for labial treatments.
The pending U.S. Patent application of T.O.P. Service fuer Lingualtechnik GmbH (“T.O.P. Service”), filed Feb. 13, 2002, Ser. No. 10/075,676, entitled “Modular System for Customized Orthodontic Appliances”, (now issued U.S. Pat. No. 6,776,614), describes a bracket system that works best with a “canted” wire. The principal axis of the cross-section of such a canted wire is oriented substantially parallel to the lingual surfaces of the teeth in order to increase patient comfort. Obviously, such a wire requires a high degree of customization.
Several attempts have been made to develop methods to customize orthodontic archwires made from SMA. For example, it has been proposed by Miura to insert the wire inside a tubular body of non-SMA material and apply the desired deformations to both the archwire and the tube. See U.S. Pat. No. 5,092,941 (Method for imparting shapes to shape memory alloy wires). The method of Miura has not been widely adopted. Manually applying precise bends and twists to a wire is difficult enough and gets significantly harder when an additional tube that encases the wire has to be deformed as well. The required forces to apply deformations increase, and since the tubular body needs to have a longitudinal slit in order to remove the archwire after heat treatment, it introduces additional unpredictable mechanical side-effects during deformation.
In U.S. Pat. No. 5,295,886 (Orthodontic archwire shaping method and archwire-segment forming templates) Wildman teaches to insert an archwire into pre-selected, generic templates in order to constrain the wire during heat treatment. Wildman's approach has several disadvantages. Firstly, his assumption that an assortment of pre-fabricated templates would cover all necessary geometric requirements for a large number of orthodontic cases seems questionable. Each patient's teeth are different, and it is highly probably that at a relevant percentage of patients, the generic templates would not cover the specific shapes needed for the patient. Especially for canted wires as referenced above, his approach is not adequate. Secondly,. Wildman's method allows customizing only the segments of the archwire between the brackets. The segments assigned to a slot remain straight. For many cases, this may be suitable, but especially for wires to be inserted in an early phase of treatment, thus the wire being required to slide through the bracket slots while the teeth are starting to align, a smooth wire shape is desirable. Especially at the section of the wire assigned to the front teeth, a smooth round curve will be advantageous. Thirdly, the process of setting up and adjusting the templates to each individual wire is laborious, costly and has high error susceptibility.
In U.S. Pat. No. 5,456,600 (Coordinated orthodontic archwires and method of making same) Andreiko teaches to produce a heat formed wire by cutting the contour of the archwire to he formed in a template. The template is then separated into two parts, and the milled surfaces are used on a plate-like clamp fixture to confine between the template parts the archwire forming wire material. Andreiko's method is obviously restricted to planar wires, since the plate-like clamp fixture does not allow for individual vertical control values at specific wire portions. Complex archwire shapes as required by the system of T.O.P. Service cannot be produced with his method.
The published PCT patent application of OraMetrix, Inc., publication no. WO 01/80761, discloses a system that, among other things, allows to bend and twist SMA wires using a six-axis-robot. The wire is shaped by two grippers, one of them attached to the robot and thus being moveable in a numerically controlled manner. The wire is being held at two adjacent locations and deformed to the desired shape, the deformation being at least partially elastic. A heat treatment is applied to the deformed portion of the wire in order to permanently retain the shape. With two grippers, a complete archwire can be shaped step by step. While this process can be highly automated, it has the disadvantages that it is time consuming since each bend can only be shaped after the previous bend has been finished; it requires heavy customized machinery; and it requires a very high amount of precision for each bend, since any systematic deviation from the desired shape adds up in the bending process, so there is little control of the overall shape. The process therefore is time-consuming and critical regarding the overall shape. Furthermore, typically a series of wires is used for orthodontic treatment, the wires showing the same shape, but being made from different cross-sections in order to provide different stiffness. Each wire from such a series has to be fabricated individually by the robot, regardless of the fact that the whole series has identical shapes.
Accordingly, a need remains for an efficient and reliable method for fabricating customized non-planar archwires made from shape memory alloys.